SCSI (Small Computer System Interface) is a well-known standard I/O bus. Standards have been established for bus widths of 8 bits and 16 bits that provide a maximum data transfer rate of 160 MB/sec. According to the standard for the 8 bit bus width, up to eight device IDs (#0 to #7) can be assigned while, according to the standard for the 16 bit bus width, up to 16 device IDs (#0 to #15) can be assigned. A host computer must also be assigned a device ID, which is typically #7. A variety of devices, including hard disk drives, CD-ROMs and scanners, can be connected to a SCSI bus.
Because of a demand for high extensibility in a computing system, there has been a great demand for connecting multiple storage devices to the system. Particularly, in an environment wherein multimedia data are frequently handled, a lot of voluminous image data may be recorded. Considering use for servers, it is desired to implement data storage of around 500 GB, preferably at a low price. Since many SCSI compatible hard disk drives have been developed, and SCSI is readily extended by using a daisy chain, a mass storage system can be constituted by connecting multiple hard disk drives to a SCSI bus.
Generally, multiple hard disk drives are housed in a hard disk enclosure. Such a hard disk enclosure is provided with a host computer interface card and a service processor for controlling the hard disk drives. Since the host computer and the service processor are also connected to the SCSI bus, they are assigned unique device IDs, respectively. For example, for a 16 bit bus width, #7 is assigned to the host computer and #15 is assigned to the service processor. Thus, for such a hard disk enclosure, excluding the device IDs assigned to the host computer and the service processor, fourteen device IDs are available so that up to fourteen hard disk drives can be connected to the SCSI bus.
However, there may be a need to connect an additional host computer to enhance redundancy, taking system reliability into consideration. The second host computer may be connected through the interface card like the first host computer, and assigned a device ID #6, for example. In such a case, if a hard disk drive mounted in a hard disk enclosure has already been assigned the same device ID #6, a device ID contention would occur. The device ID contention, if occurred, would cause a failure, e.g., data to be written to a hard disk drive by the second host computer are lost, or data recorded on the hard disk drive #6 are lost.